1. Field of the Invention
The present invention relates to an image reader for reading an image on a document sheet and in particular to an image reader which is capable of reading the images on both sides of the document sheet through single transfer of the document sheet.
2. Description of the Related Art
In the related art, an image reader (automatic double-side image reader) which reads the image information on both sides of a document sheet without the intervention of the user. Such an automatic duplex image reader most widely employs a method for inverting a document sheet in the document inverter for data reading. When image information is input with a sheet inverted, the image on the front side of the sheet is read by a specific document reader and the sheet is inverted and is delivered to the specific document reader and the image on the rear side of the sheet is read. However, this automatic duplex reading by way of sheet inversion needs to invert a document sheet and deliver the document sheet to a document reader again after temporarily ejecting the document sheet. This requires more time for reading both sides of a document sheet thus reducing the productivity of duplex reading. In an attempt to solve this problem, there is provided a technology which automatically reads both sides of a document sheet through a single pass transfer of the document sheet.
Image readers according to the related art widely employs a system where a light emitted from a fluorescent tube is irradiated onto a document and the reflected light from the document is read with an image sensor through a minification optical system in order to read the document. An image sensor using such a system is for example a one-dimensional CCD (Charge Coupled Device) sensor which simultaneously processes image read on a single line. In this system, once readout of a single line in the direction of a line (main scan direction) is over, the document sheet is traveled for a minute distance in the direction (sub scan direction) orthogonal to the main scan direction to read the next line. This procedure is repeated over the entire document size to complete the document read process over one page. A method for sequential scan in the sub scan direction without moving a document is available where a moving component such as a full-rate carriage or a half-rate carriage is used to move a plurality of mirrors to perform sequential scan in the sub scan direction.
In this read system, as mentioned above, it is necessary to orient a light source to a document and read the reflected light with a CCD sensor through a plurality of mirrors. This naturally led to a larger unit design. In particular, it is difficult, from the standpoint of limitation of space, to provide a plurality of image sensors so as to read both sides of a document sheet without inverting the sheet. In order to solve the problems with space, use of an image sensor called CIS (Contact Image Sensor) is under examination which reads an image with a linear sensor while using a small-shaped LED (Light Emitting Diode) as a light source and through for example a SELFOC lens.
In general, an LED has a sharp emission characteristic including the light-emission wavelength while the fluorescent lamp has a broad emission characteristic. As a result, in case the light source in one reading unit is a fluorescent lamp and that in the other reading unit is an LED, a difference in the emission spectrum between the fluorescent lamp and the LED presents the problem mentioned below in reading as monochrome images the color images formed on both sides of a document sheet.
For example, assume that the spectroscopic reflection spectrum of images formed in a document has a specific tendency (such as a dominant blue image). In this case, a reading unit using a fluorescent lamp as a light source can output a read signal in accordance with the density of the blue images because the emission spectrum of the fluorescent lamp contains a blue component. On the other hand, a reading unit using an LED as a light source, in case the emission spectrum of the LED contains a very small volume of blue component, cannot output a read signal in accordance with the density of the blue images but constantly outputs a read signal corresponding to a high density. Depending on the difference in the emission characteristic between the light sources obtains different densities on the front and rear sides concerning a blue image. This problem is not limited to an image having a blue component but may relate to an image with a dominant red component.